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Parallel Threading Overview

Writing simple C code trying to control output from two different threads:

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>

sem_t sem;

void* thread_func(void* aArgs)
{
  printf("Entering thread %p with %d\n", (void*)pthread_self(), (int)aArgs);
  int i = 0;
  for(;i < 10; i++)
  {
    sem_wait(&sem);
    if ((i % 2) == (int)aArgs)
      printf("val is %d in thread %p \n", i, (void*)pthread_self());
    sem_post(&sem);
  }
}

int main()
{
  pthread_t thread_1, thread_2;

  sem_init(&sem, 0, 1);

  pthread_create(&thread_1, NULL, (void*)thread_func, (void*)0);
  pthread_create(&thread_2, NULL, (void*)thread_func, (void*)1);

  pthread_join(thread_1, NULL);
  pthread_join(thread_2, NULL);

  sem_destroy(&sem);

  return 0;
}

What I want to achieve is a sequence of mixed odd and even numbers. But I get all numbers from one stream, then all other numbers from the second stream, like this (even if I increase the value of the loop counter):

Entering thread 0xb75f2b40 with 0
val is 0 in thread 0xb75f2b40 
val is 2 in thread 0xb75f2b40 
val is 4 in thread 0xb75f2b40 
val is 6 in thread 0xb75f2b40 
val is 8 in thread 0xb75f2b40 
Entering thread 0xb6df1b40 with 1
val is 1 in thread 0xb6df1b40 
val is 3 in thread 0xb6df1b40 
val is 5 in thread 0xb6df1b40 
val is 7 in thread 0xb6df1b40 
val is 9 in thread 0xb6df1b40

The question is why do two independent threads behave as if they were two sequential tasks? Why didn't the second thread take control of execution until the first thread finished everything?

I tried adding pthread_yield () to the end of the for loop, but the situation has not changed significantly: sometimes I get the expected result, sometimes as described above.

UPD. ? ?

+4
4

, . , . 1, , . .

, sem_post, sem_wait, , , ( cpu). , pthread_yield , , .

+4

JS1 :

#include <stdio.h>
#include <pthread.h>
#include <semaphore.h>

#define NUM 3

static sem_t sem[NUM];

static void *thread_func(void *args)
{
    int i;

    for (i = 0; i < 10; ++i) {
        int cur = (long)args;        /* current thread number */
        int next = (cur + 1) % NUM;  /* next thread number*/

        if ((i % NUM) != cur)
            continue;

        sem_wait(&sem[cur]); /* lock this thread semaphore */
        printf("val is %d, thread num = %ld\n", i, (long)args);
        sem_post(&sem[next]); /* unlock next thread semaphore */
    }

    return NULL;
}

int main(void)
{
    size_t i;

    pthread_t t[NUM];

    for (i = 0; i < NUM; ++i)
        sem_init(&sem[i], 0, 0); /* locked */

    for (i = 0; i < NUM; ++i)
        pthread_create(&t[i], NULL, thread_func, (void *)i);

    sem_post(&sem[0]);

    for (i = 0; i < NUM; ++i)
        pthread_join(t[i], NULL);

    for (i = 0; i < NUM; ++i)
        sem_destroy(&sem[i]);

    return 0;
}

:

val is 0, thread num = 0
val is 1, thread num = 1
val is 2, thread num = 2
val is 3, thread num = 0
val is 4, thread num = 1
val is 5, thread num = 2
val is 6, thread num = 0
val is 7, thread num = 1
val is 8, thread num = 2
val is 9, thread num = 0
+2

sem_wait sem_post , - sem_post, sem_wait ( ) .

:

pthread_mutex_t mut;
pthread_cond_t print_cond;
int print_thread; //equals 0 or 1

, . print_thread 0, , 1, , .

thread_func:

for(;i < 10; i++)
{
    pthread_mutex_lock(&mut);
    if ((i % 2) == (int)aArgs){
        while (print_thread != (int)aArgs){
            pthread_cond_wait(&print_cond, &mut);
        }
        printf("val is %d in thread %p \n", i, (void*)pthread_self());
        print_thread = 1 - (int)aArgs;
        pthread_cond_signal(&print_cond);
        pthread_mutex_unlock(&mut);
    } else {
        pthread_mutex_unlock(&mut);
    }
}

, :

Entering thread 0xb6fbcb70 with 1
Entering thread 0xb77bdb70 with 0
val is 0 in thread 0xb77bdb70 
val is 1 in thread 0xb6fbcb70 
val is 2 in thread 0xb77bdb70 
val is 3 in thread 0xb6fbcb70 
val is 4 in thread 0xb77bdb70 
val is 5 in thread 0xb6fbcb70 
val is 6 in thread 0xb77bdb70 
val is 7 in thread 0xb6fbcb70 
val is 8 in thread 0xb77bdb70 
val is 9 in thread 0xb6fbcb70

, : : print_thread.

+1

pthread_yield() - , sched_yield() sched.h.

0 sem_post .

,

for(;i < 10; i++)
{
    sem_wait(&sem);
    if ((i % 2) == (int)aArgs)
        printf("val is %d in thread %p \n", i, (void*)pthread_self());
    sem_post(&sem);
    sched_yield();
}

:

sem_init(&sem, 0, 0);

pthread_create(&thread_1, NULL, (void*)thread_func, (void*)0);
pthread_create(&thread_2, NULL, (void*)thread_func, (void*)1);

sem_post(&sem);

get:

Entering thread 0x7f74c7697700 with 0
val is 0 in thread 0x7f74c7697700 
Entering thread 0x7f74c6e96700 with 1
val is 2 in thread 0x7f74c7697700 
val is 4 in thread 0x7f74c7697700 
val is 1 in thread 0x7f74c6e96700 
val is 3 in thread 0x7f74c6e96700 
val is 5 in thread 0x7f74c6e96700 
val is 7 in thread 0x7f74c6e96700 
val is 6 in thread 0x7f74c7697700 
val is 9 in thread 0x7f74c6e96700 
val is 8 in thread 0x7f74c7697700
0

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